Many communications systems, such as systems that involve bus communications, are susceptible to (largely undesirable) high-voltage conditions, as may be present under conditions of electromagnetic interference (EMI), such as ISO pulses. This interference can raise issues with system operation.
One type of system that is susceptible to undesirable operation under EMI is an automotive type of bus communications system operated in states including a data “one” and a data “zero” state. Often, switching between states is controlled gradually, to ensure that circuits relating to each state are not concurrently active. The common voltage of the bus is often fixed using relatively high impedances (e.g., greater than 20 Kohms).
Under conditions in which the bus is subjected to EMI, the voltages on the communications system corresponding to the respective data states will either rise or fall, and may go beyond the supply limits. For example, when bus voltages rise above the supply voltage or drop below ground level, the current source (for either data “one” or “zero”) will not behave correctly. When one of the current sources deteriorates or is switched off completely, the operation of the bus is detrimentally impacted.
Under the above current deterioration conditions, any receiver monitoring the voltage on the bus is affected. For example, when the voltage on the bus becomes too low, received signals may suffer from jitter or, in extreme cases, the receiver will be unable to distinguish between data “one” and data “zero,” and data transfer may be lost.
These and other matters have presented challenges to the design and implementation of communications systems for a variety of applications.